Widespread, sustained disasters like drought, wildfire, and flooding will affect virtually every human on the planet within their lifetimes, according to a dire new assessment of the global impacts of climate change.

Within a few dozen years, many of the world’s most vulnerable communities will be forced to adjust to new realities of poverty, famine, and social unrest—decades earlier than previous timelines have predicted.

The Intergovernmental Panel on Climate Change’s report, which was published ahead of a United Nations meeting in Incheon, South Korea, says that previous efforts to limit warming to 2 degrees Celsius above pre-industrial levels will not be enough to mitigate serious impacts. The report directs policymakers to aim for a new target of 1.5 degrees—which would reduce the number of people “susceptible to poverty by up to several hundred million by 2050.”

The report calls for nothing less than a complete overhaul of the world economy, which its 132 authors agree is scientifically possible—but, for the U.S. particularly, politically unlikely.

“These systems transitions are unprecedented in terms of scale, but not necessarily in terms of speed, and imply deep emissions reductions in all sectors,” says the report.

The deadline to avert the worst effects of the crisis is coming up soon. Emissions worldwide will need to peak by 2020, meaning dramatic action needs to be taken within 15 short months.

Yet while the challenge sounds insurmountable, it’s not impossible—in 27 cities, including nine in the U.S., emissions have already peaked.